1. Field of the Invention
The present invention relates to a broadband mobile communication system, and more particularly to a method for performing a handover in a broadband wireless communication system employing an orthogonal frequency division multiplexing (hereinafter, referred to as an OFDM) method and an orthogonal frequency division multiplexing access (hereinafter, referred to as an OFDMA) method.
2. Description of the Related Art
In the fourth generation (hereinafter, referred to as a 4G) communication system, which is the next generation communication system, research has been actively pursued to provide users with services having various quality of services (hereinafter, referred to as a QoSs) and supporting a transmission speed of about 100 Mbps. The current third generation (hereinafter, referred to as a 3G) communication system supports a transmission speed of about 384 kbps in an outdoor channel environment having a relatively unfavorable channel environment, and supports a transmission speed of 2 Mbps at maximum even in an indoor channel environment having a relatively favorable channel environment.
Meanwhile, wireless local area network (hereinafter, referred to as a LAN) system, and a wireless metropolitan area network (hereinafter, referred to as a MAN) systems generally support transmission speeds of 20 to 50 Mbps. Accordingly, in the current 4G communication system, research has been actively pursued to develop a communication system to ensure mobility and QoS in the wireless LAN system and the wireless MAN system supporting relatively high transmission speeds, and to support a high speed service which will be provided by the 4G communication system.
Since the wireless MAN system has a wide service coverage and supports a high transmission speed, it is suitable for supporting a high speed communication service. However, because the wireless MAN system is a system which does not completely consider the mobility of a subscriber station (SS), a handover due to the high-speed movement of a subscriber station is also not considered. Herein, the wireless MAN system is a broadband wireless access (BWA) communication system, and it has a service coverage area which is larger than that which the wireless LAN system provides and, additionally, supports a transmission speed higher than that which the wireless LAN system provides.
A system employing an OFDM method and an OFDMA method in order to enable a physical channel of the wireless MAN system to support a broadband transmission network is known as an IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system. The IEEE 802.16a communication system is a broadband wireless access communication system employing an OFDM/OFDMA method. Further, since the IEEE 802.16a communication system applies an OFDM/OFDMA method to the wireless MAN system, the IEEE 802.16a communication system transmits a physical channel signal using a plurality of sub-carriers, thereby enabling a high speed data transmission. Therefore, the IEEE 802.16a communication system is called a broadband wireless access communication system.
Hereinafter, the structure of the conventional IEEE 802.16a communication system will be descried with reference to FIG. 1.
FIG. 1 is a block diagram schematically showing the structure of the IEEE 802.16a communication system. The IEEE 802.16a communication system has a single cell structure and includes a base station (BS) 100 and a plurality of subscriber stations 110, 120, and 130 controlled by the base station 100. The transmission/reception of signals between the base station 100 and the subscriber stations 110, 120, and 130 are accomplished through the OFDM/OFDMA method.
Hereinafter, the structure of the downlink frame of the conventional IEEE 802.16a communication system will be descried with reference to FIG. 2 which is a view which illustrates the structure of the downlink frame of the IEEE 802.16a communication system.
Referring to FIG. 2, the downlink frame of the IEEE 802.16a communication system includes a preamble portion 200, a broadcast control portion 210, and a plurality of time division multiplex (hereinafter, referred to as a TDM) portions 220 and 230. A synchronization signal (i.e., preamble sequence) used in obtaining a mutual synchronization between a base station and a subscriber station is transmitted through the preamble portion 200. The broadcast control portion 210 includes a downlink (hereinafter, referred to as a DL) portion DL-MAP 211 and an uplink (hereinafter, referred to as an UL) portion UL-MAP 213. The DL_MAP portion 211 is a portion through which a DL_MAP message is transmitted. Table 1 shows information elements (hereinafter, referred to as IEs) contained in the DL_MAP message.
TABLE 1SyntaxSizeNotesDL_MAP_Message_Format( ) {Management Message Type=2 8 bitsPHY Synchronization FieldVariableSee Appropriate PHYspecificationDCD Count 8 bitsBase Station ID48 bitsNumber of DL_MAP Element n16 bitsBegin PHY specific section {See Applicable PHYsectionfor (i=1; i<=n; i++) {For each DL_MAPelement 1 to nDL_MAP Information Element( )VariableSee correspondingPHY specificationIf!(byte boundary) { 4 bitsPadding to reachPadding Nibblebyte boundary    }   }  } }
As shown in Table 1, the DL_MAP message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘PHYsical (PHY) Synchronization’ set according to a modulation method and a demodulation method applied to a physical channel in order to obtain a synchronization, the ‘DCD count’ representing a count corresponding to the configuration variation of a downlink channel descript (hereinafter, referred to as a DCD) message containing a downlink bust profile, the ‘Base Station ID’ representing a base station identifier (BSID), and the ‘Number of DL_MAP Elements n’ representing the number of elements existing after the Base Station ID. Especially, the DL_MAP message contains information on ranging codes assigned to each ranging which will be described later.
Further, the UL_MAP portion 213 is a portion through which an UL_MAP message is transmitted. Table 2 shown below illustrates IEs contained in the UL_MAP message.
TABLE 2SyntaxSizeNotesUL_MAP_Message_Format( ) {Management Message Type=3 8 bitsUplink Channel ID 8 bitsUCD Count 8 bitsNumber of UL_MAP Element n16 bitsAllocation Start Time32 bitsBegin PHY specific section {See ApplicablePHY sectionfor (i=1; i<=n; i++) {For each UL_MAPelement 1 to nUL_MAP_Information_Element( )VariableSee correspondingPHY specification     }    }   }
As shown in Table 2, the UL_MAP message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘Uplink Channel ID’ representing a used uplink channel identifier, the ‘UCD count’ representing a count corresponding to the configuration variation of an uplink channel descript (hereinafter, referred to as an UCD) message containing an uplink bust profile, and the ‘Number of UL_MAP Elements n’ representing the number of elements existing after the UCD count. Herein, the uplink channel identifier is uniquely assigned in a media access control (hereinafter, referred to as a MAC) sub-layer.
Further, the TDM portions 220 and 230 (as shown in FIG. 2) are portions corresponding to time slots assigned to each subscriber station by a time division multiplexing (hereinafter, referred to as a TDM) time division multiple access (hereinafter, referred to as a TDMA) TDM/TDMA method. The base station transmits broadcast information, which must be broadcasted, to subscriber stations managed by the base station through the DL_MAP portion 211 of the downlink frame by means of a preset center carrier. Then, each of the subscriber stations is powered on and the base station then monitors all frequency bands set in each of the subscriber stations in advance, and detects a pilot channel signal having the highest pilot carrier to interference and noise ratio (hereinafter, referred to as a CINR).
Also, the subscriber station determines a base station having transmitted the pilot channel signal having the highest CINR to be a base station to which the subscriber station currently belongs. Further, the subscriber station confirms the DL_MAP portion 211 and the UL_MAP portion 213 of the downlink frame transmitted by the base station, and confirms control information controlling an uplink and a downlink of the subscriber station and information representing an actual position of data transmission/reception.
Table 3, illustrated below, shows the structure of the UCD message.
TABLE 3SyntaxSizeNotesUCD-message_Format( ) { Management Message Type=08 bits Uplink Channel ID8 bits Configuration Change Count8 bits Mini-slot size8 bits Ranging Backoff Start8 bits Ranging Backoff End8 bits Request Backoff Start8 bits Request Backoff End8 bits TLV Encoded Information for the overall channelVariable Begin PHY Specific Section {   for(i=1; i<=n; I+n) {    Uplink_Burst_DescriptorVariable   }  }}
As shown in Table 3, the UCD message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘Uplink Channel ID’ representing a used uplink channel identifier, the ‘Configuration Change Count’ counted by a base station, the ‘Mini-slot Size’ representing the size of a mini-slot of an uplink physical channel, the ‘Ranging Backoff Start’ representing a start point of a backoff using an initial ranging, that is, the size of an initial backoff window using an initial ranging, the ‘Ranging Backoff End’representing an end point of a backoff using an initial ranging, that is, the size of a final backoff window, the ‘Request Backoff Start’ representing a start point of a backoff for ‘contention data and requests’, that is, the size of an initial backoff window, and the ‘Request Backoff End’representing an end point of a backoff for ‘contention data and requests’, that is, the size of a final backoff window.
Herein, the value of the backoff represents a kind of waiting time value for which a subscriber station must wait for the next ranging when failure occurs in rangings which will be described later. Further, a base station must transmit the backoff value, which is information on a time period for which the subscriber station must wait for the next ranging, to the subscriber station when the subscriber station fails in a ranging. For instance, when a value by the Ranging Backoff Start and the Ranging Backoff End is set to be 10, the subscriber station passes a chance in which the subscriber station can perform rangings of 210 times (i.e., 1024 times) and then must perform the next ranging.
Hereinafter, the structure of the uplink frame of the conventional IEEE 802.16a communication system will be descried with reference to FIG. 3, which is a view which illustrates the structure of the uplink frame of the IEEE 802.16a communication system.
Before describing FIG. 3, rangings as described and used in the IEEE 802.16a communication system, include, an initial ranging, a maintenance ranging, that is, a periodic ranging, and a bandwidth request ranging each of which will be described in detail below.
The initial ranging is a ranging which is performed when a base station requests the initial ranging in order to obtain a synchronization with a subscriber station. Further, the initial ranging is a ranging which is performed in order to match an exact time offset between the subscriber station and the base station and adjust the transmit power. That is, the subscriber station is powered on, receives a DL_MAP message, an UL_MAP message, and a UCD message, and obtains synchronization with the base station. Then, the subscriber station performs the initial ranging to adjust the time offset and the transmit power with the base station. The base station receives the MAC address of the subscriber station from the subscriber station through the initial ranging procedure. Further, the base station generates a basic connection ID (hereinafter, referred to as a basic CID) and a primary management connection ID (hereinafter, referred to as a primary management CID) mapped with the received MAC address of the subscriber station, and then transmits the generated basic CID and the primary management CID to the subscriber station. Then, the subscriber station recognizes the basic CID and the primary management CID of the subscriber station through the initial ranging procedure.
Herein, since the IEEE 802.16a communication system employs an OFDM/OFDMA method, the ranging procedure requires ranging sub-channels and ranging codes. A base station assigns usable raging codes (RCs) according to the object of a raging, that is, the kind of a raging. This will be described in detail.
The raging code is generated by segmenting pseudo-random noise (hereinafter, referred to as a PN) sequence having a predetermined length (e.g., length of 215−1 bits) by a predetermined unit. Generally, two sub-channels having a length of 53 bits constitute one ranging channel. Further, the raging code is constructed by segmenting a PN code through the ranging channel having a length of 106 bits. The 48 raging codes i.e., RC#1 to RC#48) (at maximum for 48 ranging codes per subscriber station) constructed in this way may be assigned to a subscriber station, and two raging codes (at minimum per a subscriber station) are applied to the three types of rangings, that is, the initial ranging, the periodic ranging and the bandwidth request ranging, according to a default value. In this way, different raging codes are assigned to each ranging. For instance, N number of raging codes are assigned for the initial ranging (N RCs for initial ranging), M number of raging codes are assigned for the periodic ranging (M RCs for periodic ranging), and L number of raging codes are assigned for the bandwidth request ranging (L RCs for BW-request ranging). The raging codes assigned in this way are transmitted to subscriber stations through the DL_MAP message as described above, and the subscriber stations perform the ranging procedure by using the raging codes contained in the DL_MAP message according to the objects of the raging code.
The periodic ranging is a ranging periodically performed when the subscriber station having adjusted the time offset and the transmit power with the base station through the initial ranging adjusts a channel status, etc., with the base station. The subscriber station performs the periodic ranging by means of the ranging codes assigned for the periodic ranging.
The bandwidth request ranging is a ranging performed when the subscriber station having adjusted the time offset and the transmit power with the base station through the initial ranging requests a bandwidth assignment in order to actually perform a communication with the base station.
Referring to FIG. 3, the uplink frame includes an ‘Initial Maintenance Opportunities’ portion 300 using the initial ranging, and the maintenance ranging, that is, the periodic ranging, a ‘Request Contention Opportunities’ portion 310 using the bandwidth request ranging, and a ‘SS scheduled data’ portion 320 containing the uplink data of subscriber stations. The Initial Maintenance Opportunities portion 300 includes a plurality of access burst intervals actually containing an initial ranging and a periodic ranging, and a collision interval in which collision between access burst intervals occurs. The Request Contention Opportunities portion 310 includes a plurality of bandwidth request intervals containing a bandwidth request ranging, and a collision interval in which collision between bandwidth request intervals occurs. Further, the SS scheduled data portion 320 includes a plurality of SS scheduled data parts (i.e., SS 1 scheduled data part to SS N scheduled data part) and a subscriber station transition gap which is present in each of the SS scheduled data parts.
An uplink interval usage code (hereinafter, referred to as a UIUC) portion which is a portion in which information designating the use of an offset recorded in an offset portion is recorded. Table 4 below shows the UIUC portion.
TABLE 4ConnectionIE nameUIUCIDDescriptionReserved0NAReserved for future useRequest 1anyStarting offset of requestregionInitial 2broadcastStarting offset of maintenanceMaintenanceregion (used in Initial Ranging)Station 3unicastStarting offset of maintenanceMaintenanceregion (used in periodic Ranging)Data Grant 4unicastStarting offset of Data GrantBurst Type 1Burst Type 1 assignmentData Grant 5unicastStarting offset of Data GrantBurst Type 2Burst Type 2assignmentData Grant 6unicastStarting offset of Data GrantBurst Type 3Burst Type 3 assignmentData Grant 7unicastStarting offset of Data GrantBurst Type 4Burst Type 4 assignmentData Grant 8unicastStarting offset of Data GrantBurst Type 5Burst Type 5 assignmentData Grant 9unicastStarting offset of Data GrantBurst Type 6Burst Type 6 assignmentNull IE10zeroEnding offset of the previousgrant. Used to bound the lengthof the last actual intervalallocationEmpty11zeroUsed to schedule gaps intransmissionReserved12 to 15N/AReserved
As shown in Table 4, the UIUC portion contains information designating the use of the offset recorded in the offset portion. For instance, when a value of 2 is recorded in the UIUC portion, it signifies that a starting offset used in the initial ranging is recorded in the offset portion. When a value of 3 is recorded in the UIUC portion, it signifies that a starting offset used in the maintenance ranging or the bandwidth request ranging is recorded in the offset portion. As described above, the offset portion is a portion recording a starting offset value used in the initial ranging, the bandwidth request ranging, or the maintenance ranging according to the information recorded in the UIUC portion. Further, information on a characteristic of a physical channel to be transmitted in the UIUC portion is recorded in the UCD message.
Hereinafter, a ranging process between a base station and a subscriber station in the conventional IEEE 802.16a communication system will be descried with reference to FIG. 4 which is a flow diagram illustrating the ranging process between the base station and the subscriber station in the IEEE 802.16a communication system.
Referring to FIG. 4, the subscriber station 400 is powered on, monitors all frequency bands set in the subscriber station 400 in advance, and detects a pilot channel signal having the highest CINR. Also, the subscriber station 400 determines a base station 420 having transmitted the pilot channel signal having the highest CINR to be the base station 420 to which the subscriber station 400 currently belongs. Then, the subscriber station 400 receives the preamble of the downlink frame transmitted from the base station 420 and obtains a system synchronization with the base station 420.
As described above, when the system synchronization is obtained between the subscriber station 400 and the base station 420, the base station 420 transmits a DL_MAP message and an UL_MAP message to the subscriber station 400 in steps 411 and 413, respectively. Herein, as described in Table 1, the DL_MAP message performs the function of informing the subscriber station 400 of information required when the subscriber station 400 obtains a synchronization with respect to the base station 420 in a downlink, and information on the structure of a physical channel capable of receiving messages transmitted to the subscriber station 400 in the downlink. Further, as described in table 2, the UL_MAP message performs the function of informing the subscriber station 400 of information on the scheduling period of a subscriber station and the structure of a physical channel in an uplink.
Meanwhile, the DL_MAP message is broadcasted from a base station to all subscriber stations. Herein, a case in which a certain subscriber station can continuously receive the DL_MAP message signifies that the subscriber station has synchronized with the base station. That is, the subscriber stations having received the DL_MAP message can receive all messages transmitted through a downlink. Further, as described in Table 2, when the subscriber station fails in an access, the base station transmits the UCD message containing information notifying an usable backoff value to the subscriber station.
Meanwhile, when the subscriber station 400 having synchronized with the base station 420 performs the ranging, the subscriber station 400 transmits a ranging request (hereinafter, referred to as a RNG_REQ) message to the base station 420 in step 415. Then, in step 417, the base station 420 having received the RNG_REQ message transmits a ranging response (hereinafter, referred to as a RNG_RSP) message, which contains information for compensating for frequency, time, and transmit power for the ranging, to the subscriber station 400.
Table 5 illustrated below, shows the structure of the RNG_REQ message.
TABLE 5SyntaxSizeNotesRNG_REQ_message_Format( ) { Management Message Type=48 bits Downlink Channel ID8 bits Pending Until Complete8 bits TLV Encoded InformationVariableTLV specific}
As shown in Table 5, the ‘Downlink Channel ID’ represents a downlink channel identifier contained in the RNG_REQ message received in the subscriber station 400 through the UCD. The ‘Pending Until Complete’ represents a priority of a transmitted ranging response. That is, when the Pending Until Complete has a value of 0, a previous ranging response has a high priority. In contrast, when the Pending Until Complete has values other than 0, a currently transmitted ranging response has a high priority.
Table 6 illustrated below, shows the structure of the RNG_RSP message Corresponding to the RNG_REQ message shown in Table 5.
TABLE 6SyntaxSizeNotesRNG_RSP_message_Format( ) { Management Message Type=58 bits Uplink Channel ID8 bits TLV Encoded InformationVariableTLV specific}
As shown in Table 6, the ‘Uplink Channel ID’ represents an uplink channel identifier contained in the RNG_REQ message. Meanwhile, in FIG. 4, since the IEEE 802.16a communication system considers only a state in which a subscriber station is currently motionless, that is, it does not entirely consider the mobility of the subscriber station, the base station 420 (as shown in FIG. 4) communicating with the subscriber station 400 unconditionally becomes a serving base station.
As described above, the IEEE 802.16a communication system considers only a state in which a subscriber station is currently motionless (i.e., a state in which the mobility of the subscriber station is not entirely considered), and a single cell structure. However, an IEEE 802.16e communication system is stipulated as a system considering the mobility of a subscriber station in the IEEE 802.16a communication system. Accordingly, the IEEE 802.16e communication system must consider the mobility of a subscriber station in a multi-cell environment. In order to support the mobility of the subscriber station in a multi-cell environment, changes in operations of the subscriber station and a base station are necessarily required. Especially, in order to support the mobility of the subscriber station, research into a handover of the subscriber station considering a multi-cell structure has been actively pursued.
Hereinafter, the structure of the conventional IEEE 802.16e communication system will be described with reference to FIG. 5.
FIG. 5 is a block diagram schematically showing the structure of the IEEE 802.16e communication system.
Referring to FIG. 5, the IEEE 802.16e communication system has a multi-cell structure, that is, a cell 500 and a cell 550. Further, the IEEE 802.16a communication system includes a base station 510 controlling the cell 500, a base station 540 controlling the cell 550, and a plurality of mobile subscriber stations (MSSs) 511, 513, 530, 551, and 553. The transmission/reception of signals between the base stations 510 and 540 and the mobile subscriber stations 511, 513, 530, 551, and 553 is accomplished through an OFDM/OFDMA method. Herein, the mobile subscriber station 530 (of the mobile subscriber stations 511, 513, 530, 551, and 553) exists in an overlapping area (i.e., handover area) between the cell 500 and the cell 550. Accordingly, only when a handover for the mobile subscriber station 530 must be supported, it is possible to support the mobility for the mobile subscriber station 530.
In the IEEE 802.16e communication system, a certain mobile subscriber station receives pilot channel signals transmitted from a plurality of base stations, and measures CINRs of the received pilot channel signals. The mobile subscriber station then selects a base station, which is the base station that has transmitted a pilot channel signal having the highest CINR from among the measured CINRs of the pilot channel signals, as a base station to which the mobile subscriber station currently belongs. That is, the mobile subscriber station recognizes a base station, which transmits a pilot channel signal capable of being favorably received in the mobile subscriber station, from among base stations having transmitted pilot channel signals as a base station to which the mobile subscriber station belongs. As a result, the base station to which the mobile subscriber station currently belongs becomes a serving base station. The mobile subscriber station having selected the serving base station receives a downlink frame and an uplink frame transmitted from the serving base station. Herein, the downlink frame and the uplink frame of the IEEE 802.16e communication system have the same structures as those of the downlink frame and the uplink frame of the IEEE 802.16a communication system shown in FIGS. 2 and 3 above and described herein.
The serving base station transmits a mobile subscriber station neighbor advertisement (hereinafter, referred to as a MOB_NBR_ADV) message to the mobile subscriber station. Table 7 illustrated below shows the structure of the MOB_NBR_ADV message.
TABLE 7SyntaxSizeNotesMOB_NBR_ADV_message_Format( ) { Management Message Type=48 8 bits Configuration Change Count 8 bits N_NEIGHBORS 8 bits For (j=0;j< N_NEIGHBORS;J++){   Neighbor BS-ID48 bits   Physical Frequency32 bits   TLV Encoded Neighbor InformationVariableTLV specific }}
As shown in Table 7, the MOB_NBR_ADV message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘Configuration Change Count’ representing the number of times by which a Configuration changes, the ‘N_NEIGHBORS’ representing the number of neighbor base stations, the ‘Neighbor BS-ID’ representing identifiers (ID) of the neighbor base stations, the ‘Physical Frequency’ representing the physical frequency of the neighbor base station, and the ‘TLV Encoded Neighbor Information’ representing extra information relating to the neighbor base station in addition to the information.
The mobile subscriber station having received the MOB_NBR_ADV message transmits a mobile subscriber station scanning interval allocation request (hereinafter, referred to as a MOB_SCN_REQ) message to the serving base station when the mobile subscriber station intends to scan the CINRs of pilot channel signals transmitted from neighbor base stations. Herein, since a time point at which the mobile subscriber station requests a scanning has no direct relation to a scanning operation for the CINR of the pilot channel signal, a detailed description about the time point will be omitted.
Table 8 illustrated below shows the structure of the MOB_SCN_REQ message.
TABLE 8SyntaxSizeNotesMOB_SCN_REQ_message_Format( ) { Management Message Type=? 8 bits Scan Duration16 bitsUnits are frames}
As shown in Table 8, the MOB_SCN_REQ message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message and the ‘Scan Duration’ representing a scan duration for which the mobile subscriber station scans the CINRs of the pilot channel signals transmitted from the neighbor base stations. The ‘Scan Duration’ is constructed by the frame. Herein, the ‘Management Message Type’ of the MOB_SCN_REQ message to be transmitted has not been defined yet (i.e., Management Message Type=undefined or “?” as is shown in the table to conserve space).
Meanwhile, the serving base station having received the MOB_SCN_REQ message transmits a mobile subscriber station scanning interval allocation response (hereinafter, referred to as a MOB_SCN_RSP) message, which contains information to be scanned by the mobile subscriber station, to the mobile subscriber station. Table 9 illustrated below shows the structure of the MOB_SCN_RSP message.
TABLE 9SyntaxSizeNotesMOB_SCN_RSP_message_Format( ) { Management Message Type=? 8 bits Length 8 bitsin bytes For(I=0;i<Length/3;i++){   CID16 bitsbasic CID of the MSS   Duration 8 bitsin frames }}
As shown in Table 9, the MOB_SCN_RSP message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the connection ID (hereinafter, referred to as a CID) of the mobile subscriber station having transmitted the MOB_SCN_REQ message, and a scan duration. In Table 9, the ‘Management Message Type’ of the MOB_SCN_RSP message to be transmitted has not been defined yet (i.e., Management Message Type=undefined), and the scan duration is a duration for which the mobile subscriber station performs the pilot CINR scanning. The mobile subscriber station having received the MOB_SCN_RSP message containing the scanning information scans pilot CINRs for neighbor base stations, which has been recognized through the MOB_NBR_ADV message, according to the scanning information parameters.
In order to support a handover in the IEEE 802.16e communication system, a mobile subscriber station must measure CINRs of pilot channel signals transmitted from neighbor base stations and a base station (i.e., serving base station) to which the mobile subscriber station currently belongs. Further, when the CINR of the pilot channel signal transmitted from the serving base station is smaller than the CINRs of the pilot channel signals transmitted from the neighbor base stations, the mobile subscriber station requests a handover from the serving base station. Herein, for convenience of description, the sentence ‘measure the CINR of the pilot channel signal’ may be expressed by a sentence ‘scan or perform a scanning for the CINR of the pilot channel signal’. Also, the words ‘scan’ and ‘scanning’ have the same concept; ‘scan’ is used together with ‘scanning’ for convenience of description.
Hereinafter, a handover process by the request of a mobile subscriber station in the conventional IEEE 802.16e communication system will be descried with reference to FIG. 6.
FIG. 6 is a flow diagram illustrating the handover process by the request of the mobile subscriber station in the conventional IEEE 802.16e communication system.
Referring to FIG. 6, first, a serving base station 640 transmits a MOB_NBR_ADV message to a mobile subscriber station 600 in step 611. Then, the mobile subscriber station 600 receives the MOB_NBR_ADV message and obtains information on neighbor base stations. Further, in step 613, the mobile subscriber station 600 transmits a MOB_SCN_REQ message to the serving base station 640 when the mobile subscriber station 600 intends to scan the CINRs of pilot channel signals transmitted from the neighbor base stations. Herein, since a time point at which the mobile subscriber station 600 requests a scanning has no direct relation to a scanning operation for the CINR of the pilot channel signal, a detailed description about the time point will be omitted. Meanwhile, in step 615, the serving base station 640 having received the MOB_SCN_REQ message transmits the MOB_SCN_RSP message, which contains information to be scanned by the mobile subscriber station 600, to the mobile subscriber station 600. In step 617, the mobile subscriber station 600 having received the MOB_SCN_RSP message containing the scanning information performs a scanning for the CINRs of pilot channel signals with respect to neighbor base stations, which have been recognized through the reception of the MOB_NBR_ADV message, according to parameters (i.e., scan duration) contained in the MOB_SCN_RSP message.
Next, after having completely scanned the CINRs of the pilot channel signals received from the neighbor base stations, when the mobile subscriber station 600 determines to change the serving base station 640 to which the mobile subscriber station 600 currently belongs in step 619, that is, the mobile subscriber station 600 determines to change the current serving base station 640 to another new base station, the mobile subscriber station 600 transmits a mobile subscriber station handover request (hereinafter, referred to as a MOB_MSSHO_REQ) message to the serving base station 640 in step 621. Herein, a new base station (i.e., a base station to which the mobile subscriber station 600 is to be handed over), which is not a serving base station to which the mobile subscriber station 600 currently belongs, is called a target base station (target BS). Table 10 shows the structure of the MOB_MSSHO_REQ message and is illustrated below.
TABLE 10SyntaxSizeNotesMOB_MSSHO_REQ_message_Format( ) { Management Message Type=52 8 bits N_Recommended 8 bits For (j=0;j< N_NEIGHBORS;J++){   Neighbor BS-ID48 bits   BS S/(N+1) 8 bits   Service level prediction 8 bits }}
As shown in Table 10, the MOB_MSSHO_REQ message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, and the ‘N_Recommended’ representing a result obtained by a scanning of a mobile subscriber station. Herein, as shown in Table 10, the ‘N_Recommended’ contains the identifiers of neighbor base stations, a CINR of a pilot channel signal for each of the neighbor base stations, and the level of a service predicted to be provided from the neighbor base stations to the mobile subscriber station.
Meanwhile, when the serving base station 640 receives the MOB_MSSHO_REQ message transmitted from the mobile subscriber station 600, the serving base station 640 detects a list of target base stations to which the mobile subscriber station 600 can be handed over by means of the ‘N_Recommended’ information of the received MOB_MSSHO_REQ message in step 623. Herein, for convenience of description, the list of target base stations to which the mobile subscriber station 600 can be handed over will be called a ‘handover-executable target base station list’. In FIG. 6, it is assumed that a first target base station 660 and a second target base station 680 exist in the handover-executable target base station list. Also, the handover-executable target base station list may include a plurality of target base, stations. In steps 625 and 627, the serving base station 640 transmits a handover notifications (hereinafter, referred to as HO_notifications) message to the target base stations (i.e., the first target base station 660 and the second target base station 680) contained in the handover-executable target base station list. Table 11 shows the structure of the HO_notification message and illustrated below.
TABLE 11SyntaxSizeNotesGlobal Header152-bitFor (j=0;j< Num Records;J++){  MSS unique identifier 48-bit48-bit unique identifier used byMSS (as provided by the MSS orby the I- am-host-of message)  Estimated Time to HO 16-bitIn milliseconds, relative to thetime stamp, value 0 of thisparameter indicates that no actualHO is pending  Required BW 8-bitBandwidth which is required byMSS (to guarantee minimumpacket data transmission)  Required QoS 8-bitName of Service Classrepresenting AuthorizedQoSparamSet } Security fieldTBDA means to authenticate thismessage CRC field 32-bitIEEE CRC-32
As shown in Table 11, the HO_notification message includes a plurality of IEs, that is, an identifier MSS ID of the mobile subscriber station 600 intending to perform a handover procedure to the first target base station 660 or the second target base station 680, an estimated start time of a handover by the mobile subscriber station 600, and information on the bandwidth requested from the mobile subscriber station 600 to a target base station which will become the new serving base station, and the level of a service that is to be provided to the mobile subscriber station 600. Herein, the bandwidth and the service level requested by the mobile subscriber station 600 are identical to the predicted service level information recorded in the MOB_MSSHO-REQ message described in Table 10 above.
Meanwhile, when the first target base station 660 or the second target base station 680 receive the HO_notification messages from the serving base station 640, they each transmit handover notification response (hereinafter, referred to as a HO_notification_response) messages, response messages with respect to the HO_notification message, to the serving base station 640 in steps 629 and 631, respectively. Table 12 shows the structure of the HO_notification_response message and is illustrated below.
TABLE 12SyntaxSizeNotesGlobal Header152-bitFor (j=0;j< Num Records;J++){  MSS unique identifier 48-bit48-bit unique identifier used byMSS (as provided by the MSS orby the I- am-host-of message)  QoS Estimated 8-bitBandwidth which is provided byBS(to guarantee minimum packetdata transmission)TBD how to setthis field  BW Estimated 8-bitQuality of Service levelUnsolicited Grant Service (UGS)Real-time polling Service (rtPS)Non-Real-time polling ServicenrtPS)Best Effort  ACK/NACK 1-bitAcknowledgement or Negativeacknowledgement1 is Acknowledgement whichmeans that the neighbor BSaccepts the HO_notificationmessage from the serving BS0 is Negative Acknowledgementwhich means that the neighbor BSmay not accept theHO_notification message fromthe serving BS  }  Security fieldTBDA means to authenticate thismessage  CRC field 32-bitIEEE CRC-32
As shown in Table 12, the HO_notification_response message includes a plurality of IEs, that is, an identifier MSS ID (MSS unique identifier) of a mobile subscriber station intending to perform a handover procedure to target base stations, a response ACK/NACK regarding whether or not the, target base stations can perform a handover according to the handover request of the mobile subscriber station, and bandwidth and service level information capable of being provided by each target base station when the mobile subscriber station is handed over to each target base station.
Meanwhile, the serving base station 640 having received the HO_notification_response messages from the first target base station 660 and the second target base station 680 analyzes the received HO_notification_response message, and selects a target base station, which can optimally provide the bandwidth and the service level requested by the mobile subscriber station 600 when the mobile subscriber station 600 is handed over, as a final target base station to which the mobile subscriber station 600 is to be handed over. For instance, when it is assumed that the service level capable of being provided by the first target base station 660 is smaller that that requested by the mobile subscriber station 600, and the service level capable of being provided by the second target base station 680 is identical to that requested by the mobile subscriber station 600, the serving base station 640 selects the second target base station 680 as a final target base station to which the mobile subscriber station 600 is to be handed over. Accordingly, the serving base station 640 transmits a handover notification confirmation (hereinafter, referred to as a HO_notification_confirm) message, a response message with respect to the HO_notification_response message, to the second target base station 680 in step 633. Table 13 shows the structure of the HO_notification_confirm message and is illustrated below.
TABLE 13SyntaxSizeNotesGlobal Header152-bitFor (j=0;j< Num Records;J++){  MSS unique identifier 48-bit48-bit universal MAC address ofthe MSS (as provided to the BSon the RNG-REQ message)  QoS Estimated 8-bitBandwidth which is provided byBS(to guarantee minimum packetdata transmission)TBD how to setthis field  BW Estimated 8-bitQuality of Service levelUnsolicited Grant Service (UGS)Real-time polling Service (rtPS)Non-Real-time polling Service(nrtPS)Best Effort}Security fieldTBDA means to authenticate thismessageCRC field 32-bitIEEE CRC-32
As shown in Table 13, the HO_notification_confirm message includes a plurality of IEs, that is, an identifier MSS ID (MSS unique identifier) of a mobile subscriber station intending to perform a handover procedure to a selected target base station, and bandwidth and service level information capable of being provided by the selected target base station when the mobile subscriber station is handed over to the selected target base station.
Also, the serving base station 640 transmits a mobile subscriber station handover response (hereinafter, referred to as a MOB_HO_RSP) message, a response message with respect to the MOB_MSSHO_REQ message, to the mobile subscriber station 600 in step 635. Herein, the MOB_HO_RSP message contains information on a target base station to which the mobile subscriber station 600 is to be handed over. Table 14 illustrated below, shows the structure of the MOB_HO_RSP message.
TABLE 14SyntaxSizeNotesMOB_HO_RSP_message_Format( ) { Management Message Type=53 8 bits Estimated HO time 8 bits N_Recommended 8 bits For (j=0;j< N_NEIGHBORS;J++){  Neighbor BS-ID48 bits Service level prediction 8 bitsThis parameter exists onlywhen the message is sentby the BS }}
As shown in Table 14, the MOB_HO_RSP message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, an estimated start time of a handover procedure, and the ‘N_Recommended’ representing a result for target base stations selected by a serving base station. Herein, as shown in Table 14, the ‘N_Recommended’ contains identifiers of the selected target base stations and the level of a service predicted to be provided from each target base station to a mobile subscriber station. In FIG. 6, the MOB_HO_RSP message finally includes only target base station information on the second target base station 680 from among target base stations existing in the handover-executable target base station list. However, when there exist a plurality of target base stations capable of providing bandwidth and service level requested by the mobile subscriber station 600 from among the target base stations existing in the handover-executable target base station list, the MOB_HO_RSP message includes information on the plurality of target base stations.
Next, the mobile subscriber station 600 having received the MOB_HO_RSP message analyzes the ‘N_Recommended’ information contained in the MOB_HO_RSP message and selects a target base station to which the mobile subscriber station 600 is to be handed over. Then, the mobile subscriber station 600 having selected the target base station to which the mobile subscriber station 600 is to be handed over transmits a mobile subscriber station handover indication (hereinafter, referred to as a MOB_HO_IND) message, a response message with respect to the MOB_HO_RSP message, to the serving base station 640 in step 637. Table 15 illustrated below, shows the structure of the MOB_HO_IND message.
TABLE 15SyntaxSizeNotesMOB_HO_IND_message_Format( ) { Management Message Type=54 8 bits TLV Encoded InformationVariableTLV specific Target_BS_ID48 bits}
As shown in Table 15, the MOB_HO_IND message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘Target_BS_ID’ representing an identifier of a target base station selected by a mobile subscriber station, and the ‘TLV Encoded Information’ representing extra information in addition to the information.
Meanwhile, in step 639, the serving base station 640 having received the MOB_HO_IND message recognizes that the mobile subscriber station 600 is handed over to the target base station (i.e., the second target base station 680) contained in the MOB_HO_IND message, and then releases a link currently setup with the mobile subscriber station 600. In this way, when the link with the serving base station 640 is released, the mobile subscriber station 600 performs a handover procedure to the second target base station 680 in step 641.
Hereinafter, a handover process by the request of a base station in the conventional IEEE 802.16e communication system will be described with reference to FIG. 7.
FIG. 7 is a flow diagram illustrating the handover process by the request of the base station in the conventional IEEE 802.16e communication system.
Before describing FIG. 7, the handover process by the request of the base station occurs when the base station is overloaded and requires some type of load-sharing for dispersing the load of the base station, or the base station must cope with the change of the uplink status of a mobile subscriber station.
Referring to FIG. 7, first, a serving base station 740 transmits a MOB_NBR_ADV message to a mobile subscriber station 700 in step 711. Then, the mobile subscriber station 700 receives the MOB_NBR_ADV message and obtains information on neighbor base stations. Further, in step 713, the mobile subscriber station 700 transmits a MOB_SCN_REQ message to the serving base station 740 when the mobile subscriber station 700 intends to scan the CINRs of pilot channel signals transmitted from the neighbor base stations. Herein, since a time point at which the mobile subscriber station 700 requests a scanning has no direct relation to a scanning operation for the CINR of the pilot channel signal, a detailed description about the time point will be omitted. In step 715, the serving base station 740 having received the MOB_SCN_REQ message transmits the MOB_SCN_RSP message, which contains information to be scanned by the mobile subscriber station 700, to the mobile subscriber station 700. In step 717, the mobile subscriber station 700 having received the MOB_SCN_RSP message containing the scanning information performs a CINR scanning of pilot channel signals with respect to neighbor base stations, which has been recognized through the reception of the MOB_NBR_ADV message, according to parameters (i.e., scan duration) contained in the MOB_SCN_RSP message.
Meanwhile, when the mobile subscriber station 700 managed by the serving base station 740 determines to perform a handover procedure by its own handover necessity in step 719, the serving base station 740 transmits the HO_notification messages to neighbor base stations 760 an 780 in steps 721 and 723. Herein, the HO_notification message contains information on a bandwidth and the level of a service which must be provided by a target base station to be a new serving base station of the mobile subscriber station 700. In FIG. 7, it is assumed that the neighbor base stations of the serving base station 740 are two base stations, that is, the first target base station 760 and the second target base station 780.
In steps 725 and 727, the first target base station 760 and the second target base station 780 receive the HO_notification messages respectively, and transmit the HO_notification_response messages, response messages for the HO_notification messages, to the serving base station 740. As described in Table 12, the HO_notification_response message contains a response ACK/NACK regarding whether or not the target base stations can perform a handover procedure requested by the serving base station 740, and bandwidth and service level information capable of being provided to the mobile subscriber station 700.
Next, after receiving the HO_notification_response messages from the first target base station 760 and the second target base station 780, the serving base station 740 then selects target base stations capable of providing the bandwidth and the service level requested by the mobile subscriber station 700. For instance, when it is assumed that the service level capable of being provided by the first target base station 760 is smaller than that requested by the mobile subscriber station 700, and the service level capable of being provided by the second target base station 780 is identical to that requested by the mobile subscriber station 700, the serving base station 740 selects the second target base station 780 as a final target base station to which the mobile subscriber station 700 is to be handed over. Further, the serving base station 740 having selected the second target base station 780 as the final target base station transmits a HO_notification_confirm message, which is a response message for the HO_notification_response message in step 729.
Next, after transmitting the HO_notification_confirm message to the second target base station 780, the serving base station 740 transmits a MOB_HO_RSP message to the mobile subscriber station 700 in step 731. Herein, the MOB_HO_RSP message contains N_Recommended information selected by the serving base station 740, that is, selected target base stations e.g., in FIG. 7, the second target base station 780) and the bandwidth and the service level capable of being provided from the target base stations to the mobile subscriber station 700. The mobile subscriber station 700 having received the MOB_HO_RSP message recognizes the a handover has been requested by the serving base station 740, and selects a final target base station to which the mobile subscriber station 700 is to be handed over with reference to the N_Recommended information contained in the MOB_HO_RSP message. After selecting the final target base station, the mobile subscriber station 700 transmits a MOB_HO_IND message, a response message for the MOB_HO_RSP message, to the serving base station 740 in step 733. Then, in step 735, the serving base station 740 having received MOB_HO_IND message recognizes that the mobile subscriber station 700 is to be handed over to the target base station contained in the MOB_HO_IND message, and then releases a link currently setup with the mobile subscriber station 700. In this way, when the link with the serving base station 740 is released, the mobile subscriber station 700 performs a handover procedure to the second target base station 780 in step 737.
As described above, in the currently proposed handover procedure in the IEEE 802.16e communication system, a serving base station collects information of neighbor base stations, transmits a HO_notification message, and collects information necessary for the handover. Then, the serving base station receives a HO_notification response message as a response of the HO_notification message, and transmits a MOB_HO_RSP message containing information of target base stations to which a mobile subscriber station can be handed over to the corresponding mobile subscriber station. Meanwhile, the mobile subscriber station determines a base station to which the mobile subscriber station is to be handed over from a list of target base stations, to which the mobile subscriber station can be handed over, contained in the MOB_HO_RSP message, and then transmits a MOB_HO_IND message containing information on the determined base station to the serving base station. Then, the mobile subscriber station releases a current connection with the serving base and tries to connect with the determined base station.
As described above, only a simple procedure for a handover is defined up to now. However, various circumstances may exist which have been not described in the procedure in a broadband mobile communication service of actual various radio environments. For instance, there may occur a case in which the serving base station must forcedly cause the subscriber station to perform a handover procedure according to the resource condition of the serving base station, or special circumstances such as the rejection of the subscriber station for a handover requested by the serving base station. Also, there may occur a case in which the movement direction of the subscriber station changes while being handed over to a target base station, and the subscriber station must cancel the handover procedure in order to connect to an original serving base station again.
However, the conventional broadband mobile communication system has not yet presented methods for solving the circumstances which may occur as described above. Additionally, when the conventional method is applied to the aforementioned circumstances, it is ineffective and further, the performance of the system may be substantially reduced.